The present invention relates generally to the field of signal amplification circuitry, such as circuitry used in medical diagnostic systems, and stability techniques used to enhance performance of such amplification circuitry. More particularly, the invention relates to a technique for reducing input impedance of a preamplifier circuit, such as a preamplifier in a magnetic resonance imaging system to reduce crosstalk between signals originating in phased array and other coils.
Magnetic resonance imaging systems have found increasing applicability for a variety of imaging tasks, particularly in the medical field. Such systems typically include coil assemblies for generating magnetic fields used to control and excite gyromagnetic materials in a subject of interest, such as in soft tissues of a patient. A body coil is typically employed for generating a highly uniform magnetic field along a principal axis of the subject. A series of gradient coils generate spatially varying magnetic fields to select a portion of the subject to be imaged, and to spatially encode sensed signals emitted by unitary volumes within the selected slice. The gradient fields may be manipulated to orient the selected image slice, and to perform other useful imaging functions.
Sensing coils are employed in conventional MRI systems and are adapted to the particular type of image to be acquired. Such sensing coils are highly sensitive to emissions from the subject positioned within the primary and gradient fields. Such emissions, collected during data acquisition phases of imaging, serve to generate raw data signals which may be processed to extract information relating to the nature and location of gyromagnetic material in the subject. Where the region to be imaged is relatively small, a single channel surface coil may be employed. For example, a linear shoulder coil is typically employed for producing images of a human shoulder. For larger images, large single coils may be employed, or multiple coils may be used, such as in xe2x80x9cphased arrayxe2x80x9d arrangements. However, the use of large surface coils tends to result in lower signal-to-noise ratios in the acquired image data. Phased array coil assemblies are, therefore, commonly employed to produce images of larger areas, while providing an acceptable signal-to-noise ratio.
Signals acquired by surface coils in MRI systems are typically amplified in one or more preamplifier circuits prior to further signal processing. For example, in phased array coil systems, output signals from each of several adjacent coils are independently amplified in the preamplifiers prior to processing of the signals for generation of the image data. In a typical phased array arrangement, several adjacent coils are provided for receiving the signals emitted by the gyromagnetic material during the signal acquisition phase of imaging. A problem in such systems arises from crosstalk between adjacent coils. To limit or reduce such crosstalk, one common approach is to overlap adjacent coils in the system. Due to the current-carrying paths established by each coil, such overlapping reduces or cancels mutual inductive coupling between the coils, thereby reducing crosstalk. However, such overlap techniques are not always feasible, depending upon the coil configuration.
Another technique for reducing crosstalk in multi-channel imaging coils involves the provision of an LC matching network and a preamplifier. In this technique, a high resistance to induced current flow in coils in receiving mode is provided by the LC network connected to the preamplifier. To provide the maximum resistance to such induced current, the input impedance of the preamplifier must be kept to a minimum. In existing systems of this type, small input impedances, on the order to 2-5 ohms are typical. However, even such low impedance levels are not sufficient for certain multi-channel coil structures, such as multi-channel brain coils. Thus, while the LC matching approach is generally preferable to the overlapping coil technique, further reduction in the input impedance for the preamplifiers used in such imaging systems is still needed.
The invention provides a novel technique for reducing the input impedance for a preamplifier, such as for use in a magnetic resonance imaging system designed to respond to this need. The technique permits the input impedance of the preamplifier circuit to be reduced to a level of substantially zero. The circuitry providing the input impedance adjustment may permit imaginary and real components of the input impedance to be adjusted independently. Accordingly, the imaginary component of the input impedance may be adjusted to a substantially zero level, followed by subsequent adjustment of the real component. The circuitry conveniently includes a feedback circuit wherein a solid state amplification device is coupled between the amplifier input and output nodes. The feedback circuit has a capacitance level which is balanced by adjustment of a feedback control circuit. The circuitry may be coupled to a reactance matching circuit and reduces the input impedance of the amplifier.